[PATCH] (struct re_pattern_buffer): reduce by cache line

Reducing struct re_pattern_buffer from 464 to 448 bytes allows it to fit
into neatly into seven 64-byte cache lines on x86-64 systems.

ABI is changed in 2.2.0dev anyways, so we may do this.
Will commit in a few days.
C extension users will need to rebuild extensions for this.
There'll probably be more changes like this (rb_io_t, maybe others).